Inkjet printers are well known in the art. Small droplets of liquid ink, propelled by thermal heating, piezoelectric actuators, or some other mechanism, are deposited by a printhead on a print media, such as paper.
In scanning-carriage inkjet printing systems, inkjet printheads are typically mounted on a carriage that is moved back and forth across the print media. As the printheads are moved across the print media, a control system activates the printheads to deposit or eject ink droplets onto the print media to form text and images. The print media is generally held substantially stationary while the printheads complete a “print swath”, typically an inch or less in height; the print media is then advanced between print swaths. The need to complete numerous carriage passes back and forth across a page has meant that inkjet printers have typically been significantly slower than some other forms of printers, such as laser printers, which can essentially produce a page-wide image.
The ink ejection mechanisms of inkjet printheads are typically manufactured in a manner similar to the manufacture of semiconductor integrated circuits. The print swath for a printhead is thus typically limited by the difficulty in producing very large semiconductor chips or “die”. Consequently, to produce printheads with wider print swaths, other approaches are used, such as configuring multiple printhead die in a printhead module, such as a “page wide array”. Print swaths spanning an entire page width, or a substantial portion of a page width, can allow inkjet printers to compete with laser printers in print speed.
Using multiple printhead die in a printhead assembly can create other problems, however. While the physical spacings of the ink ejection mechanisms (or “nozzles”) in a single die are determined by the semiconductor manufacturing steps, which are extremely precise, the spacing between nozzles in different die within a module are subject to slight misalignments. Further, the aerodynamic effects on ink droplets ejected by nozzles near the end of a printhead die may be different than the aerodynamic effects on ink droplets ejected nearer the center of the die. These and other factors can cause visible print defects on the printed media corresponding to the boundaries between die. These print defects generally take the form of light or dark bands or streaks on the page.
Inkjet printers often utilize multiple-pass print modes to improve print quality. By applying only a portion of the total ink on each pass, less liquid is applied to page at each pass, minimizing color bleed due to mixing of inks at color boundaries and buckling or “cockle” of the print media. Multiple print passes also allow greater optical densities to be achieved in the final print. In a drum printer, each “pass” may constitute a revolution of the drum; additional revolutions may be used for drying of the printed page. Multiple pass printing typically takes longer than single pass printing, but print quality can be substantially improved.
There is a need for methods that reduce visible print defects in images produced by multiple die printhead assemblies.